<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" 
        content="width=device-width, 
        user-scalable=no, 
        initial-scale=1.0, 
        maximum-scale=1.0, 
        minimum-scale=1.0"/>
    <title>Document</title>
    <style type="text/css">
        html{
            font-size: 10px;
        }
        /* p{
            font-size: 1em; / * 12px * /
        }
        span{
            font-size: 2em; / * 20px * /
        } */
        p{
            font-size: 1rem;/* 12px */
        }
        span{
            font-size: 2rem;
        }
    </style>

</head>
<body>
    <p>
        这是段落
        <span>段落中的文本</span>
    </p>
</body>
</html>